Grapevine Response to Soil Temperature: Xylem Cytokinins and Carbohydrate Reserve Mobilization from Budbreak to Anthesis

Potted Shiraz grapevines, in a glasshouse, were exposed to two different soil temperatures (13°C and 23°C) to evaluate the effects on vegetative growth and floral development from dormancy to anthesis. Soil temperature had no effect on the time of budbreak, anthesis, or the number of flowers per inflorescence. At anthesis total biomass was similar for both treatments, whereas shoot biomass was greater in the warm soil. From dormancy to anthesis, both root and trunk biomass decreased in the cool soil and only root biomass decreased in the warmer soil, but by twice as much as that in the cool soil. During dormancy to anthesis decreases in total nonstructural carbohydrate accounted for most of the decrease in root biomass. At budbreak, 14 cytokinins representing four recognized classes were present in bleeding sap, with trans-zeatin riboside and isopentenyl adenosine as the dominant forms. Total and active free base cytokinin concentrations were similar for both treatments, while sap from vines in the warm soil had significantly lower concentrations of nucleotide cytokinins. However, delivery of cytokinins was significantly greater in the warm soil treatment. By anthesis, cytokinin concentrations were similar for both treatments, but total cytokinin concentrations in xylem sap had decreased by almost 90% from budbreak. Root-generated cytokinins appear to be associated with the mobilization of the carbohydrate reserves at the end of dormancy and the ensuing shoot growth. Comparison of results with those of previous studies reveals that, because of apical dominance and correlative inhibition, the response to soil temperature in terms of number of buds to break and time of budbreak is conditioned by the number of nodes per cane.